CAPE (Convective Available Potential Energy) is available convective potential energy air. This is the amount of energy that an air parcel can gain as it rises in the atmosphere. A higher CAPE value means that the air parcel has more buoyancy force – it rises faster and more intensely. In other words, the higher the CAPE, the greater the instability air and thus stronger convective processes (cumulus clouds and storms). CAPE is calculated from temperature and humidity profiles (from aerological soundings or meteorological models) and is expressed in joules per kilogram (J/kg). Higher CAPE therefore means more “fuel” for a storm: it supports strong updrafts, the formation of massive cumulus clouds and potentially the formation of storms with torrential rain, wind, hail or sleet.

CAPE and storm activity

Meteorologists often look at the CAPE value when forecasting storms. A typical CAPE value scale corresponds to the level of storm instability:

• 0–500 J/kg: low value – the air is relatively stable, storms are unlikely.
• 500–1500 J/kg: moderate instability – possible thunderstorm formation, especially conditions for common showers.
• 1500–3000 J/kg: high instability – stronger storms with heavy rainfall or hail are likely.
• above 3000 J/kg: very high to extreme instability – capacity for very strong storms, supercell storms and tornadoes.

Higher CAPE provides more energy to storms. For example, a CAPE value of around 1000 J/kg is usually sufficient for strong or potentially destructive storms. Extreme values above 3000–4000 J/kg indicate a very turbulent atmosphere in which storms with enormous force and destructive gusts can form. In practice, however, CAPE alone does not guarantee a storm – it is only the “fuel”, a trigger (front, cold updraft) and usually significant crosswind shift (eddy shear) are also needed, which determines whether the storm remains isolated (supercell) or spreads into lines.

Extreme weather: hail, floods, lightning

Higher CAPE increases risk extreme weather. Strong convective storms with high CAPE often bring heavy rain and flash floods, strong winds, and especially large hail. For example, in the summer of 2023, northern Italy was hit by two intense supercell storms that produced hailstones of enormous size: on July 19, hailstones up to 16 cm in diameter and on July 24, even a record 19 cm. These are the largest hailstones measured in European history. These storms damaged thousands of cars and homes, injured hundreds of people - strong CAPE supported the formation of giant hailstones and extreme cloudbursts. Similarly, storms with high CAPE also hit other areas - for example, supercell storms resulted in an IF3 tornado in northern Italy and caused numerous floods during the summer of 2023.

In addition to hail, CAPE storms also produce heavy downpours. The high-energy atmosphere can accumulate enormous amounts of water vapor in a short time, which then falls as torrential rain. These storms can cause local flooding or flash floods. For example, the flood in Derna, Libya (September 2023) had a complex background, but the extreme storm Daniele, combined with high convective potential, led to catastrophic downpours; similarly, the summer floods in Greece and Bulgaria in 2023 were accompanied by a very convective atmosphere.

CAPE and climate change

With global warming, it also increases average energy of the atmosphere. Warmer air can hold more moisture and is naturally “saturated” with energy. Various studies and real-world observations show that as temperatures rise, the number of days with high CAPE also increases. For example, an analysis of data from the eastern United States for the period 1979–2021 showed that in recent decades there has been an increase in days with high CAPE, mainly in spring and summer. Similarly, in Europe, long-term measurements confirm a significant increase in CAPE in the decades leading up to the end of the 20th century. This increase was associated with an increase in humidity near the ground – higher temperatures over the middle of the Mediterranean Sea bring more moisture into the air, which increases CAPE and with it the risk of large hail and severe storms. Model studies further predict that at temperatures 2–3 °C higher, we can expect even more frequent occurrence of very high CAPE values. This also corresponds to the observation that damage caused by hail and storms has increased sharply in recent years – for example, in Europe, storms with large CAPE caused billions in damage (e.g. in Italy in 2023 about €6 billion).

Therefore, due to climate change, probability of extreme storms. Lower layer stability and higher humidity in warm months create more frequent high-CAPE environments. This trend is expected to continue in the future, according to studies – model projections predict an increase in the presence of high-CAPE conditions, which could lead to more frequent intense storms even in a changing climate. At the same time, it appears that future storms may be fewer but more intense – that is, isolated storm days may bring increasingly strong storms thanks to higher CAPE.

CAPE in weather forecasting

Meteorologists use CAPE as one of the key indicators of storm risk. Numerical weather models calculate CAPE from predicted atmospheric temperature and humidity, and experts analyze it together with other parameters (such as wind shear, CIN, LS parameter, etc.). In practice, this means that if the model predicts air with a CAPE value of, for example, 2000 J/kg, meteorologists expect an increased probability of thunderstorms with hail or torrential rain. Conversely, a low CAPE indicates a more stable atmosphere and calmer weather.

Additionally, research shows that CAPE helps improve forecasting intensity of storm events – higher CAPE, together with other factors (e.g. wind shear), allows us to identify environments where the strongest supercell storms or tornadoes could form. Therefore, CAPE is commonly displayed on forecast maps and is part of climate and storm indices. For example, gridded models of the European Medium-Range Weather Forecast Service (ECMWF) and American numerical forecast systems regularly forecast CAPE over Europe to warn of possible convective storms.

CAPE is a fundamental meteorological parameter for storm activity. It expresses energy available to the air for storm formation and is directly correlated with storm intensity (with precipitation, hail, and wind). As global temperatures rise, so does CAPE—and with it the risk of sudden torrential rains, tornadoes, and record-breaking hail. Early understanding of this parameter helps us better predict and prepare for storms brought on by a changing climate.

Sources: The data and claims were drawn from meteorological studies and analyses (NOAA, Climate Central, ESSL, iMeteo, etc.) that document the link between high CAPE and severe storms or climate change. Each claim is supported by a citation to the relevant scientific literature or a professional meteorological website. Spring